Now that the tools target requires the generated version header file, we
need to make sure that the directory it writes to exists.  In a configured
tree, this is taken care of for us.  But in an unconfigured one, the dir
does not yet exist causing a build error like so:

/bin/sh: line 5: ..../u-boot_build/include/version_autogenerated.h.tmp: No such file or directory

So create the dir for this file before we attempt to generate it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>